The present invention is in the field of measurement and inspection techniques, and relates to a focusing assembly and method to be used in a charged particle beam column.
Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor wafers, being utilized in a CD metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope) and defect review SEM. SEM typically includes such main constructional parts as an electron beam source and an electron beam column. The electron beam column comprises inter alia a focusing assembly and a detection unit. The focusing assembly typically includes a lens arrangement and a deflector. The detection unit may comprise an appropriate number of detectors for detecting secondary electrons and/or back-scattered electrons. A wafer under inspection is located on a stage under the electron beam column. Such a SEM is disclosed, for example, in U.S. Pat. No. 5,502,306.
One of the common goals of all imaging systems consists of increasing the image resolution. In SEM, in order to reduce the xe2x80x9cspotxe2x80x9d size of the electron beam up to nanometers, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Specifically, the electron optic elements are more effective (i.e. produce smaller aberrations) when the electrons are accelerated to high kinetic energy. However, it has been observed that such a highly energized electron beam causes damage to resist structures and integrated circuits, and, in the case of dielectrical specimens, causes undesirable charging of the specimen.
One prior art technique for focusing charged particles on a specimen, which is aimed at solving the above problem, utilizes the introduction of a retarding electrostatic immersion lens in the path of a focused beam of accelerated charged particles, so as to decelerate them prior to hitting the sample. The provision of the retarding field in the path of a deflected focused beam reduces the aberration of focusing and deflecton. For further relevant information about this technique, the disclosure in U.S. Pat. No. 4,926,054 can be referred to.
Deceleration of the electrons can generally be accomplished by selectively creating a potential difference between the pole piece of a magnetic objective lens and the specimen. Alternatively, the same effect can be achieved by actually introducing electrodes having selective potential applied thereto.
However, due to the low-energy of the beam having been decelerated, dispersion of the beam is observed. Chromatic aberration of focusing and deflection is more severe in the low-energy beam than in the high-energy beam.
Thus, one of the main problems of scanning electron microscopy is associated with the following. A high-energy electron beam is required for focusing purposes, and deflection of this beam is required for scanning purposes (i.e., to illuminate a certain scan area). Ideally, all the electrons of the primary electron beam generated inside a column have the same energy. However, in practice, some variation in energy is present among the electrons. This variation degrades the image in two ways. First, when electrons pass through a lens, the focusing effect of the lens depends on the individual electron""s energy. Hence, electrons having different energies are affected by the lens differently. This causes chromatic aberration (the energy of the electrons is analogous to wavelength of light beam in optical systems). In the second way, when charged particles pass through a deflector, the effect of the deflecting field depends on the energy of the electron. Therefore, electrons of different energies are deflected to different extents, and, consequently, chromatic aberration of deflection is added to the unavoidable chromatic aberration of focusing. Chromatic aberration of deflection is proportional to the angle of deflection. Hence, image quality deteriorates with the distance from the center of the scan area. Chromatic aberration of deflection increases with the reduction of beam energy.
Another known problem of the inspection systems of the kind specified is associated with locating defects (foreign particles) on patterned surfaces. The pattern is typically in the form of a plurality of spaced-apart grooves. To detect the existence of a foreign particle located inside a narrow groove, it is desirable to tilt the scanning beam with respect to the surface, which tilting should be applied to selective locations on the specimen. As proposed in U.S. Pat. No. 5,734,164, a tilt mechanism is implemented by mechanically tilting the wafer carrier relative to the charged particle beam column. The maximum angle of beam incidence on the wafer reaches sixty degrees. It is needless to say that such a mechanical tilting of the specimen carrier, as well as the mechanical tilting of the column as disclosed in U.S. Pat. No. 5,329,125, is difficult to implement and is time consuming.
There is accordingly a need in the art to improve conventional scanning electron microscopy by providing a novel focusing assembly and method to be used in a charged particle beam column.
It is a major feature of the present invention to provide such a system that allows for the non-destructive inspection of specimens with high quality imaging. Non-destructive inspection is an inspection that causes no physical damage to the specimen under inspection.
It is a futher feature of the present invention to provide such a system that facilitates the inspection and measurements of patterned specimens, such as semiconductor wafers, masks, reticles, etc.
The main idea of the present invention is based on the use of first and second deflectors, wherein the first deflector is accommodated in a focusing field of a lens arrangement, and the second deflector is accommodated in the focusing field in the path of a charged particle beam deflected by the first deflector, and wherein the second deflector operates in a predetermined suitable mode with respect to that of the first deflector. By this, chromatic aberrations of deflection are substantially reduced. Moreover, the provision of the first and second deflectors enables a certain average (at least small) angle of incidence of the charged particle beam to be provided with reduced chromatic aberrations. The tilting of the incident charged particle beam is achieved by adjusting the operational mode of the second deflector with respect to that of the first deflector. As indicated above, the tilting of the incident beam is desired, for example, to detect defects that cannot be detected with the normal incidence of the charged particle beam, namely when operating with the normal deflection mode. Such a defect may, for example, be a particle located on the inner surface of the side wall of a narrow groove.
To reduce the chromatic aberration of deflection with the normal incidence of the charged particle beam (xe2x80x9cnormal deflection modexe2x80x9d), the first and second deflectors deflect the beam in the same direction. In this case, an electric field in the vicinity of the specimen may be substantially low, i.e., in a range (xe2x88x92100-1000)V/mm.
To provide the desired tilting of the incident beam (the so-called xe2x80x9ctiltxe2x80x9d mode), the first and second deflectors may deflect the beam either in two opposite directions, namely the first deflector deflects the beam away from the axis of beam propagation, and the second deflector deflects the beam towards the axis, or in the same direction, away from the axis of beam propagation. In this case, an electric field in the vicinity of the specimen is preferably substantially high, as compared to that of the normal deflection mode, e.g., is in a range 1-5 kV/mm. By appropriately operating the first and second deflectors deflecting the beam in two opposite directions, the beam can be applied to the same point as with the normal deflection mode, thereby providing the so-called xe2x80x9con-axis tiltxe2x80x9d.
Thus, according to one aspect of the present invention, there is provided a focusing assembly for directing a charged particle beam onto a specimen, the focusing assembly comprising:
(a) a lens arrangement producing a focusing field;
(b) a first deflector accommodated within the focusing field and operable with a preset mode to provide beam deflection in a predetermined direction; and
(c) a second deflector accommodated downstream of the first deflector with respect to the direction of beam propagation, the second deflector being operable with a predetermined mode with respect to the operational mode of the first deflector.
The lens arrangement includes an objective magnetic lens composed of two pole pieces, and may include a retarding electrostatic immersion lens producing the electric field. Alternatively, the electric field may be produced by the magnetic objective lens. Preferably, the first deflector is magnetic, while the second deflector may be either electrostatic or magnetic.
When the first deflector is of a magnetic type and is accommodated within the gap of the magnetic objective lens, rotation of the deflected charged particle beam unavoidably occurs. To compensate for such a rotation effect produced by the first deflector and the magnetic objective lens, the plane of deflection of the second deflector should be rotated accordingly, i.e., by the same angle of rotation. This is achieved by creating a xe2x80x9crotation fieldxe2x80x9d by the first deflector.
The focusing assembly may be used in any charged particle beam column.
According to another aspect of the present invention, there is provided a method for focusing a charged particle beam whilst directing it onto a specimen.
According to yet another aspect of the present invention, there is provided a charged particle beam column comprising:
a charged particle beam source; and
a focusing assembly for directing the charged particle beam onto a specimen comprising:
a lens arrangement producing a focusing beam;
a first deflector accommodated within the focusing field and operable with a preset mode to provide beam deflection in a predetermined direction; and
a second deflector accommodated downstream of the first deflector with respect to the direction of beam propagation, the second deflector being operable with a predetermined mode with respect to the operational mode of the first deflector.
According to yet another aspects of the present invention a method and system are provided for inspecting a specimen utilizing a focusing assembly constructed as described above.
The use of the first and second deflectors enables stereoscopic imaging of a specimen. To this end, the direction fields of the first and second deflections should be selectively reversed. Thus, according to yet another aspect of the present invention, there is provided a method for imaging a specimen by a charged particle beam, the method comprising the steps of:
generating said charged particle beam and directing it towards the specimen;
generating a focusing field for focusing the beam onto the specimen;
generating a first deflection field within said focusing field for deflecting the beam in a predetermined direction;
generating a second deflection field downstream of the first deflection field with respect to the direction of beam propagation, said second deflection field deflecting the beam deflected by the first deflection field in a predetermined manner with respect to the first deflection; and
selectively reversing the direction fields of the first and second deflections, thereby obtaining stereoscopic images of the specimen.
The charged particle beam many be an electron beam or a focused ion beam (FIB). The present invention may be used in a SEM or the like tool applied to a specimen, e.g., a semiconductor wafer, for imaging, measurements, metrology, inspection, defect review or the like purposes. For example, the present invention may be used for CD measurements, line profile measurements, copper-interconnects inspection/measurements typically performed after a photolithography process, automatic defect classification, etc.
More specifically, the present invention is used with SEM system for inspecting wafers, masks or reticles, and is therefore described below with respect to this application.